Anyone's guess is as good as mine, but I'd say she's none of the 5 mentioned.

- Doubtful she's a Time Lord or The Rani. The Doctor and the TARDIS would know almost instantly.

- Prisoner Zero I thought was pretty well established to be the snaky thing from the 1st story.

- Future Amy Pond. Possible I suppose, but Amy will have a lot of maturing to do if this is the same character.

- Doctor's wife. Also remotely possible, but I think we're supposed to believe they are close to each other in The Doctor's future. She knows him well...they've been on lots of adventures together...she can drive the TARDIS...etc. I believe Moffat's setting this up to throw us a curve, because:

With River's character I don't think who she is, is as important as who she was - specifically who she killed as was hinted in the Weeping Angels story this season.

My working theory is that she's the bridge between the 11/12th Doctors. Specifically Matt Smith is the person she killed and is the reason she was in prison. So the wheels have been put in motion for Smith's regeneration into the series' next Doctor. It's great timey-wimey stuff that'll play out over the next several seasons. The Doctor's first time they met was her last...and the first time River meets The Doctor will be his last.

I could be wrong, but either way I think it'll be a while before we know her true identity.

Queue this (or next) Season's Finale where the Doctor dies at her hand (for some terribly noble reason). Only for the cracks in time (or some other Cosmic reset switch) to indo this and everyone can be friends again.

Queue this (or next) Season's Finale where the Doctor dies at her hand (for some terribly noble reason). Only for the cracks in time (or some other Cosmic reset switch) to indo this and everyone can be friends again.

None of the above... although I think the closest guess is the Doctor's wife.

As far as the others:

2) Future Amy Pond - First of all, there's the looks. I'd be disappointed if a human regenerated. Then, there's personality - they don't act alike. And yes personality can change, but I wouldn't consider it a satisfacctory resolution (I think Steven Moffat is better than that). And finally, I'm also convinced that Rory is coming back - I think future Amy would have future Rory too.

3) Time Lord - The Doctor "knows" that there are no other Time Lords, the same as he knows the Universe is turning. The reason he couldn't sense it with the Master is because A) he was human, as in Utopia, or B) technically dead (an then brought back), as in End of Time

4) Prisoner Zero - Prisoner Zero simply has much too small a role in the greater scope of the show to be River. Plus, like with Amy, it's just sort of a personality thing. We don't know much about Prisoner Zero, but we should know enough to put together that a close and possibly romantic relationship with the Doctor isn't exactly it's Modus Operandi

5) The Rani - same as the Time Lord thing, with a little bit more too. I would honestly be apt to believe this if we went just on Time of Angels/Flesh and Stone, but we can't forget the first time we saw River - somehow I doubt that the Rani would die for the Doctor. Not to mention, what became of her passion for science?

My best guess is that she's "sort of" the Doctor's wife... or maybe, would have become his wife if it wasn't for some yet-to-be-unveiled reason. I really like the idea of Romana, but what I said about being a Time Lord discludes that! :(

I also don't think the Doctor is the man she killed. We know Matt Smith has signed on for a second season, so I can't imagine in what sense she could think she killed him.

Afterthought: this just came to me - I wonder if she is "sort-of" Romana. Maybe River is the result of a meta-crisis from Romana, like the "Doctor" that stayed with Rose in Journey's End.

The Doctor's Wife makes the most sense from the list above just because of the way their back-and-forth is depicted, especially by her.

I suppose one could always surmise that since Romana stayed in e-Space when we last saw her, she was somehow shielded from the Time Wars. They did have a good relationship in the series and she had researched the Doctor's bakground before going on the search for the Key to Time. It is possible she stumbled across some old, dusty record of his real name. As far as we know, he never told her his name during their time together.

I still have a hard time with the androgynous Doctor, which is why I can't personally accept the idea that River Song is some future regeneration or variation of the Doctor himself. I just don't think the Doctor's gender would change in a regeneration. But it is Sci-Fi/Fantasy and anything is possible. Maybe the Doctor will have gender-reassignment surgery some day.

I think any known blood relative is out of the question like a sister, mother, aunt, niece, etc., because you would think the Doctor would recognize a blood relative even after they regenerate.

But then again there is always Jenny who he has only seen once and thinks is dead. They had a bit of that kind of back-forth bantering in her one appearance. But I think she would have called him "Dad" right away rather than "Sweetie."

I don't think River Song is any of the five, and I don't think the Doctor is the man she killed either. But I can't wait to find out! I love the River Song character, and I'm looking forward to seeing more of her, so in that sense I hope we don't find out too soon, because I love the mystery

both these are posibilities, however because of the marriage part im going for His and Omega's wife, last seen in the antimatter universe with former husband omega, whom she believed dead possibly sent to bring the doctor to the antimatter universe so that omega can finally escape as he needs timelord DNA to build a new body because he exists only as a will power in the alternate universe.

Having recently rewatched all the River Song episodes I have come to the conclusion that we have not met River Song before The Silence in the Library. I think she is a 100% Steven Moffatt creation. I think what will happen is that we are witnessing a super evil villain (along the lines of the Master) who, thanks to the Doctor, sees the error of her ways and becomes a reformed character only we are seeing it in reverse. This would add a new twist on a Doctor/Foe dynamic as it will mean that the Doctor will end up fighting (possibly on a regular basis) an enemy who he knows will become good and has more likely than not travelled with the Doctor and has become very close. He knows he will never truly defeat her and that he will also never know her as "good" ever again. I think this would be pretty interesting and potentially, a very tragic story...